Outside air infiltrating living area

[FrankP]

Hello, I just purchased a 2010 Montana 3455sa and I live in Corpus Christi Texas. We’ve been full timers at an rv park here for the past four years and this is our second rv. I’ve been getting hot air coming in from the kitchen cabinets and around the converter panel which is under the desk on mine. I cut an access panel on the back of a kitchen cabinet next to the converter panel to repair some open neutrals to the converter and when I pulled the piece out I found that there’s a pretty steady flow of outside air in that basement area. I’ve covered the heater returns on the stairs and foamed everything I could get access to but the air coming from that basement area is ridiculous. Is it coming through the heater somehow or where is it coming from. Please help. Electricity bill has doubled since we bought this.